The Yard Planner is a helpful, bird's-eye view blueprint of your yard. It is easier to plan your yard once built. You can check your tower ranges and move the buildings around from the Yard Planner view. It can also used as additional bait for incoming land attacks. One can also spot mushrooms easily with the Yard Planner (This feature does not work in the latest Yard Planner).
Note: This is an essential building if you want to make a good base, otherwise its virtually impossible to make one.
If you want to see just some of the towers' ranges but not all, you can click on "show tower ranges" and move all the towers except the ones you want to see to outside your yard. Your towers will pop back, but the ranges will stay outside, making the others easier to see.